The most accurate way of measuring the blood pressure in the heart or other internal organ of a patient is to insert a catheter having a transducer at its distal end through a blood vessel to the point of interest, but this increases the risk of blood clots and may abrade tissue near the heart. Current practice avoids these problems by coupling blood pressure at a peripheral site, such as in the wrist, to a transducer outside of the patient's body with a saline-filled lumen in a catheter. In the illustration of this technique shown in FIG. 1, an external transducer T is shown having a hollow pressure dome D that is mounted so as to apply the pressure in the dome D to a sensing surface 2 that translates the pressure into a corresponding signal on output leads L.sub.1 and L.sub.2. Excitation is applied to the transducer T via leads L.sub.3 and L.sub.4. A tube 4 that communicates with the interior of the dome D is connected via a valve V.sub.1 to a tube 6. Another tube 8 that communicates with the dome D is connected via a valve V.sub.2 to a lumen L in a catheter C, and the distal end of the catheter C is inserted into a blood vessel in the arm A of a patient P.
In use, the tubes 4, 6 and 8, the valves V.sub.1 and V.sub.2, the catheter C and the dome D are filled with a saline solution having nearly the same density as blood, and the catheter C is inserted into a blood vessel in the arm A of a patient P, e.g., in his wrist. The open end of the tube 6 is placed on a reference level indicated by the dashed line R that passes through the point at which the pressure is desired in the organ O. The valve V.sub.2 is closed and the valve V.sub.1 is opened. The signal on the leads L.sub.1 and L.sub.2 under this condition includes a first component due to the height of solution in the tube 6, in the tube 4 and in the valve V.sub.1 above the sensing surface 2 and a second component called "transducer offset" that may add to or subtract from the first that is different for each transducer. The monitor or other device to which the leads L.sub.1 and L.sub.2 are coupled is then adjusted to a reading of zero pressure. Note that an opening B in the body of the transducer T permits atmospheric pressure to reach the underside of the sensing surface 2 so as to balance the effect of atmospheric pressure at the open end of the tube 6. Instead of manually adjusting the monitor to zero, the signal on L.sub.1 and L.sub.2 may be stored in a memory for numerical subtraction. Then the valve V.sub.1 is closed and the valve V.sub.2 opened so that the blood pressure of the patient at the organ O may be measured. A change in the height of the point in the arm A where the distal end of the catheter C is located has no effect on the pressure indicated by the transducer T.
A severe difficulty with such apparatus is that the compliance of the walls of the catheter C and the transducer T, as well as the inertia of the saline solution, impairs the frequency response so that the fidelity of the signal on the leads L.sub.1 and L.sub.2 is significantly less than optimum.
In order to improve accuracy, it has been proposed that the transducer be placed in a blood vessel in the body at a site remote from the organ generating the pressure of interest. However, a new problem arises. Consider the case of a catheter inserted into the radial artery in the wrist of a human patient for the purpose of indicating the pressure in the aorta. In this situation, the transducer is advanced only a few inches beyond the patient's wrist toward the heart. If the patient elevates his arm such that the transducer is raised by 13.8 mm, the associated instrumentation will indicate a pressure decrease of 1 mm Hg. If one assumes that a patient has the ability to vary his wrist elevation by .+-.1.0 meter, then a pressure measurement error of .+-.72 mm Hg can result. One obvious solution to this problem is to strap the patient's wrist to his body at the level of the organ so as to prevent the patient from moving his arm. Even this extreme measure has its limitations since patients may roll or be rolled from side to side to prevent fluid pooling in the lungs and for other reasons. This orientation change would also change the elevation relationship between the organ and the transducer site and introduce an error.